1. Field of the Invention
The invention relates to a method and device for producing stainless steel, when the raw material in the process is a ferroalloy, advantageously ferrochromium.
2. Brief Description of the Prior Art
As main components, stainless steels contain iron and chromium. In order to produce this kind of iron-chromium alloy, i.e. ferrochromium, there can be used oxidic chromite ore, which contains considerable amounts of iron oxides in addition to chromium oxide. In order to create an alloy of iron and chromium, as is described in the JP patent application 57,002,813, there can also be used blast furnace-produced pig iron, whereto chromium is then alloyed in a separate converter. Furthermore, according to the JP patent application 49,067,811, in order to produce an alloy of iron and chromium, i.e. ferrochromium, there can be used iron and chromium ore in given proportions, and the said alloy is then smelted into ferrochromium in the presence of coke and steel scrap.
Apart from iron and chromium, stainless steels contain--depending on the desired properties--for instance nickel. The alloy ingredients are added to the ferrochromium in presmelting, which takes place prior to the production of stainless steel proper, and is carried out for example in an arc furnace. Thereafter the molten stainless steel master alloy is transferred for instance to an AOD converter generally used in stainless steel processes. In the converter, the additional components of the alloy, such as carbon and silicon, are adjusted to the desired level by means of oxygen and argon blasting.
The production of stainless steel requires a lot of energy, particularly electric energy, because the production of ferrochromium needed for stainless steel is mainly carried out in an electric furnace, such as an arc furnace. After smelting in an electric furnace, the ferrochromium must usually be cooled, because the AOD converter employed in the production of stainless steel is possibly located so far from the ferrochromium process, that the transport of molten ferrochromium is not possible. It may also be necessary to carry out the cooling even if the ferrochromium process and the AOD converter were located in the vicinity of each other, because the smelting period of ferrochromium and the AOD converter treatment are essentially different as for their duration.